Manchester United have struggled in recent years to get to grips with Manchester City, largely due to the physical presence and power of Yaya Toure.
At the Etihad this season they were defeated with ease, and new boy Marouane Fellaini, touted as the answer to the conundrum, was as useful as a lamppost placed in the middle of the pitch.
Last night though, one man showed them just how it was done.
Nemanja Matic, recently signed by Chelsea, was superb against City, and helped nullify Yaya Toure's impact in the middle.
Chelsea were of course aided by Fernandinho's absence, and Martin Demichelis being his replacement, but Matic's performance was one which stood out and was key in Chelsea pulling off a shock 1-0 win.
The Serbian completed a higher percentage of his passes than any other Chelsea player on the night (85 per cent), won four tackles and made two interceptions.
Alan Shearer summed it up on Match of the Day 2, stating: "He looked like Yaya Toure going past Yaya Toure."
City are entitled to an off day, but it should also be reminded that the game was Matic's first Premier League start.
Chelsea's £21 million signing from Benfica appears outstanding business, but for another side it's yet another glaring miss.
Manchester United were linked with signing Matic regularly, their interest reported last April by the Guardian, and at regular intervals right up until this January since then.
They could have bought him last summer and still had £6 million change left over for the fee they paid for Marouane Fellaini.
The loss to Stoke City once again exposed how top-sided United are, and weak in the central midfield areas.
They have been crying out for years for a player who can dominate games and take it to the very best in Yaya Toure.
Chelsea have just found one, and it appears the solution was staring United in the face all along. Now their search must begin again...
image: © Yoca91